Supercritical pressure recirculating boiler



Jan. 10, 1967 c. R. MIDTLYNG 3,297,004

SUPERCRITICAL PRESSURE RECIRCULATING BOILER Fil ed Aug. 25. 1965 2Sheets-Sheet 1 FIG. 2

CARL R. M/DTLYNG INVENTOR,

Jan. 10, 1967 c. R. MlDTLYNG 3,297,004

SUPERCRITICAL PRESSURE RECIRGULATING BOILER Filed Aug. 26; 1965 v 2Sheets-Sheet 2 CARL R. M/DTLYNG INVENTOR zimww United States PatentGfifice 3,297,064 SUPERQIRITHCAL PRESSURE nucmcura'rnsc BQTLER Carl R.Midtlyng, Worcester, Mass., assignor to Riley Stoker Corporation,Worcester, Mass, a corporation of Massachusetts Filed Aug. 26, 1965,Ser. No. 482,814 4 Claims. ((31. 122-406) This invention relates to asupercritical pressure recirculating boiler and, more particularly, toapparatus arranged to produce steam by the burning of fossil fuel and bybringing the products of combustion into heat exchange relationship towater.

It has become common practice to generate steam at high pressure by useof the well-known once-through forced-flow boiler. Such boilers operateeffectively not only at sub-critical but at supercritical pressures.However, a number of difficulties have arisen when a conventionalonce-through boiler is used, as compared with the use of a naturalcirculation boiler. As a result, a boiler which is a combination of thetwo and can be called a supercritical pressure recirculating boiler hasbeen evolved. In such a boiler, a variable percentage of the flow offluid through the main heat absorption section is recirculated back tothe entrance of the section. This re-circulation permits the maintenanceof adequate fluid flow through the main heat absorption section tubes atall loads. Furthermore, in starting up the boiler, it eliminatesconsiderably the need for complicated apparatus for disposing ofgenerated steam in that part of the start-up cycle before the steam isallowed to expand through the turbine. However, it has been found thatthe supercritical pressure recirculating boiler requires extremely largeand expensive pumps which absorb considerable power at all loads.Furthermore, considerable difliculty has been experienced, due to thefact that the fluid temperatures coming from the absorption section varyby considerable amounts, depending on the particular part of the boilercombustion chamber inwhich the fluids originate. These and otherdifficulties experienced in the prior art devices have been obviated ina novel manner by the present invention.

It is, therefore, an outstanding object of the invention to provide asupercritical pressure boiler in which recirculation takes place with aminimum pump capacity and power absorption by the recirculating pump.

Another object of this invention is the provision of a supercriticalpressure boiler in which natural forces are used to produce at least aportion of the recirculating power.

A further object of the present invention is the provision of asupercritical pressure boiler in which it is possible to select theoutput temperature very close to the limiting metal temperature withoutthe danger of exceeding that temperature.

It is another object of the instant invention to provide a supercriticalpressure boiler in which means is provided to assure that outputtemperatures do not exceed the design temperature.

It is a further object of the invention to provide a supercriticalpressure boiler in which a furnace peripheral wall outlet header andpiping arrangement assures that fluid to the superheater originates inwall areas having inherently higher heat absorption and fluid which isrecirculated originates in wall areas having inherently lower heatabsorption.

A still further object of this invention is the provision of asupercritical pressure boiler in which means is provided to limit fluidtemperatures in the tubes in the high heat absorption section.

With these and other objects in view, as will be apparent to thoseskilled in the art, the invention resides in the combination of partsset forth in the specification and covered by the claims appendedhereto.

The character of the invention, however, may be best understood byreference to one of its structural forms, as illustrated by theaccompanying drawings in which:

FIG. 1 is a schematic view of a boiler of a steam generating unitembodying the principles of the present invention,

FIG. 2 is a schematic view of the invention in plan view, and

FIG. 3 is a perspective view, with portions broken away of a boiler ofspecific details of the steam generatlng unit.

Referring first to FIG. 1, which best shows the general features of theinvention, the supercritical pressure boiler, indicated generally by thereference numeral 19, is shown as consisting of a main heat absorptionsection 11 having a series of vertical tubes 12 joined at the upper endto a header l3 and, at the lower end, to a header 14. The upper header13 is joined by connector tubes 15 to a header 15 connected through asuperheater 17 to a turbine (not shown).

The lower header 14 is connected by tubes '18 to a header 1& which, inturn, is connected by a downcomer 21 to a mixing chamber 22. The upperend of the mixing chamber is connected to a pipe 23 leading to the mainfeed pumps (not shown).

The header in is connected to the mixing chamber 22 by a pipe 24containing a flow-regulating valve 25. The header in is also connectedthrough a shutoff valve 26 to the suction side of a recirculating pump27, the output of which is connected through a shutoff valve 28 and acheckvalve 2-9 in series to the mixing chamber 22. The checkvalve 29 isarranged to allow flow from the pump 27 to the mixing chamber 22 but notin the opposite direction.

FIG. 2 shows in somewhat greater detail the interconnections at the topof the boiler. The steam generating unit is provided, of course, with anupper side wall header 13 and, at the opposite side, to an upper sidewall header 31. Extending between these headers transversely of thesteam generating unit is the header 16 to which are connected the topsof the tubes on the front and rear wall of the apparatus. The connectingpipes 15 join the ends of the upper side wall header 13 to the adjacentend of the header 16. Similar connecting pipes 32 connect the ends ofthe upper side wall header 31 to the adjacent end of the header 1%. Thecentral portion of the header 13 is connected by a tube 33 to the tube34 leading from the center of the header in to the superheater 17. Asimilar tube 35 connects the center of the upper side Wall header 31 tothe tube 34. Tubes 36 and 37 connect the central portion of the header16 to the tubes 33 and 35, respectively, adjacent their connection tothe tube 34. Tubes 38 and 39 connect the ends of the header 16 to thevalve 26 leading to the pump 27.

Referring now to FIG. 3, which shows the details of construction of thesteam generating unit, it can be seen that the steam generating unit itis provided with a front wall 41, a rear wall 42, and side walls 4-3 and44 forming a main combustion chamber. At the upper end of the side wall43 is the header 13 and, at the lower end, the

header 14. At the upper end of the side wall 44 is the header 31 and, atthe lower end, a header 45, these headers being joined by vertical sidewall tubes in the usual way. The tubes making up the front wall 41 andthe slde wall 42 are joined at their upper ends to the collector header16 and at their lower ends approach one another to form a lower hopper46. The other end of the tubes making up the front wall 41 terminate ina transverse header 47, while the tubes making up the rear wall 42terminate in a transverse header 43. This perspective view shows themanner in which the pipes join the ends of the header 13 to the adjacentend of the header 16. Similarly, they show the way in which the pipes 32join the ends of the header 31 to the adjacent end of the header 116.The pipe 33 joins the central portion of the header 13 to the pipe 34,while in a similar way the pipe 35 joins the center of the header 31 tothe pipe 34. Pipes 36 and 37 join the central portion of the header 16to the pipes 33 and 35, respectively, adjacent their connection to thepipe 34. The ends of the header 16 are joined by the pipes 38 and 39 tothe pump apparatus 27.

The pump apparatus 27 consists of a network including pumps 51 and 52.The pumps are connected through shutoff valves 28 and checkvalves 29 tothe mixing chamber 22 and are connected to the pipes 33 and 39 throughshutoff valves 26.

The tubes 38 and 39 are also connected to the end of the tube 24 leadingto the valve 25 and, eventually, to the mixing chamber 22. The upper endof the mixing chamber is connected by the tube 23 to main feed pumps,while the bottom end is connected by the downcomer Ell to the drum 19which, for the purposes of illustration, is shown as a spherical drum.This drum serves to feed the various lower headers. For instance, feedertubes 54 and 55 lead from the drum 19 to the header 48 at the bottom ofthe hopper. Similarly, tubes 53 and 56 lead from the drum 19 to theheader 47. A feed tube 58 lead from the drum to the header 45, while asimilar tube 57 leads to the header 14.

The operation of the apparatus will now be readily understood in view ofthe above description. Feedwater under pressure enters the systemthrough the tube 23 and passes into the mixing chamber 22 down thedowncomer 21 into the drum 1%. From there it flows through the feedertube system 18 consisting of the tubes 53, 54, 55, and 56 to the header14 as well as the headers 45, 47, and The water passes upwardly throughthe main heat absorption section 11, through the tubes 12, consisting ofthe tubes in the front wall 41, the rear wall 42, the side wall 43, andthe side wall 44. When the section 11 is operated at supercriticalpressure and temperature, there will be no sharp line of conversion fromliquid into steam. Most of the fluid in the system can be treated asphase fluid. The fluid which has been heated passes from the walls intothe headers 13, 31, and 16. Because of the fact that the combustionchamber is of a rectangular form, the amount of radiant heat received bythe tubes will be less for those tubes at the corners of the enclosurethan it will those in the central portions of the sides. Therefore, thetube 33 leaving the center of the header 13, the tube 35 leaving thecenter of the upper header 31, and the tubes 36 and 37 leaving thecenter of the header 16 receive effluent of higher than averagetemperature and feed it into the tube 34 going into the tubrine. Tubes15 lead fluid from the ends of the header 13 to the ends of the header16, while tubes 32 carry cool fluid from the ends of the header 31 tothe other end of the header 16. Then, this cooler than average fluidoriginating in the ends of headers 13 and 31 as well as the ends of theheader 16 itself are carried through tubes 33 and 39 to therecirculating system passing from the upper ends of the tubes 38 and 39into the mixing chamber 22. At low loads, the valves 26 and 28 are open,so that the pumps 51 and 52 cause fluid to pass from the tubes 38 and 39to the mixing chamber 22 where fluid joins the fluid coming from thefeed pumps through the feed line 23. At higher loads, there is less needfor the pumps 51 and 52 because the same circulation of the boiler whichserves to operate a natural circulation boiler also causes fluid to flowthrough the bypass 24 through the open valve 25, into the mixing chamber22, and down the downcomer 21 to the lower part of the boiler. Whenauto-circulation takes place through the bypass valve 25, thecheckvalves 29 prevent back flow through the pumps.

It can be seen that with the present construction the fluid forrecirculation is taken from the least heat-absorbing portion of thecombustion chamber. In addition, the arrangement requires that the fluidfrom the superheater for use by the turbine is taken from the highestheat-absorption portion of the furnace. The circulation in theabsorption section 11 is maintained at a value sufficient to assure thatthe tube metal temperature does not exceed a safe value. Thatcirculation is normally in the range from 3.5 to 6.0 feet/ second.Auto-recirculation is due to the gravity head of the downflow pipe 21which contains relatively cool, dense fluid, this head equaling theopposing gravity head in the upflow portion plus all the flowresistances in this closed circuit. Pumps 51 and 52 are connected inparallel with the pipe 24 and these pumps are of a head capacitycharacteristic which is typical of centrifugal pumps having asteeply-falling headcapacity curve. Each pump and its constant speedmotor are sized for the cold start-up condition of the boiler with thewater initially at, say, F. It is a fortunate result of an optimumcombination of pump and closed-circuit characteristics that, when theboiler is in the hot running condition, almost all of the total fluidrecirculated passes through the pumps, but over the entire load rangethe pumps operate at or near the maximum capacity zerohead point. Eachpump is virtually a floating pump for, with the motor energized, thepump is neither fluid driven nor does it develop any significant head inexcess of flow losses. Over a lower portion of the load range a smallfraction of the total recirculated fluid passes through the tube 24 andthe valve 25. This bypass is provided so that, when the pumps are shutdown and isolated, natural circulation flow through the pipe 24 willprovide at least a minimum safe fluid velocity at the tube entrances ofthe absorption section 11.

It can be seen, then, that there are certain advantages which may bederived from this specific boiler arrangement. First of all, it is notnecessary, as is ordinarily the case, to provide a pump motor which issized considerably in excess of the hot running requirements in order tomeet the power requirements when the same pump is used for cold startup.Furthermore, the hot running pump power requirement is negligible at allboiler loads. The flow resistance in the closed circuit is minimized, sothat the natural recirculation tendency is maximized. Each pump islogically a vertical shaft canned pump with water-lubricated bearings.It is desirable to avoid occasions for starting and stopping the pump,since the bearing wear occurs almost entirely when stopping or starting.The present arrangement permits having one pump in use continually atall times, from cold start-up all the way up through the highest loads.Although the pump requires negligible power while in the hot runningboiler load condition, recirculation is augmented over that which wouldoccur naturally, if the pump were deenergized and isolated. However,with this pump normally energized and passing most or all of therecirculated fluid, any reduction in normal mixed fluid volume flow ratewould encounter an opposing and compensating pumping action by theuncontrolled free running pump.

It will be understood that, by selecting the fluid for recirculationfrom portions of the main heat absorption section that produce thelowest temperature of eflluent, it is possible to reduce the averagefluid temperature in the section. This permits safer operation of theboiler. In other words, selective recirculation of cooler-than-averagefurnace wall efliuent results in a significant reduction in temperatureof eflluent from all tubes and a corresponding reduction in tube metaltemperature.

It is obvious that minor changes may be made in the form andconstruction of the invention without departing from the material spiritthereof. It is not, however, desired to confine the invention to theexact form herein shown and described, but it is desired to include allsuch as properly come within the scope claimed.

The invention having been thus described, what is claimed as new anddesired to secure by Letters Patent is:

1. A supercritical pressure boiler having a combustion chamber,comprising (a) a main heat absorption section having an inlet header atits lower end and an outlet header at its upper end, the section beinglocated in the combustion chamber and means to heat said section wherebyone part receives less heat than another part and delivers fluid oflower than average temperature to a portion of the outlet header,

(b) a recirculating circuit including a mixing device joining the saidinlet and outlet headers,

(c) a feed circuit including a main feed pump connected to the mixingdevice,

(d) a recirculating system connected in the recirculating circuit,

(e) means connecting said portion of the said outlet header receivingfluid of lower than average temperature to the mixing device, and

(f) a superheater conduit connected to a portion of the outlet headerreceiving fluid of higher than average temperature.

2. A super-critical pressure boiler as recited in claim 1, wherein therecirculating system includes two circuits arranged in parallel, eachcircuit containing a pump and a shut-01f valve following the pump, afurther shutofl? valve being provided in each circuit ahead of the pump,and a check valve being provided in each circuit.

3. A supercritical pressure boiler as recited in claim 1, wherein themixing chamber consists of an elongated vertical cylindrical housing,wherein the main feed pump is connected coaxially to the upper end ofthe housing, wherein the downcomer extends downwardly from the lower endof the housing to the lower end of the evaporator section, and whereinthe recirculating circuit is connected tangentially to the centralportion of the mixing chamber.

4. A supercritical pressure boiler, comprising (a) a main heatabsorption section forming at least part of a combustion chamber Wall,means to heat said section so that the section has a first part whichreceives relatively less transferred heat and a second part whichreceives relatively more transferred heat, so that the fluid reachingthe outlet end of the first part is at relatively low temperature andthe fluid reaching the outlet end of the second part is at relativelyhigh temperature,

(b) a recirculating circuit joining the inlet and outlet of the saidsection,

(c) a feed circuit including a main feed pump connected to the inlet ofthe section,

(d) means connecting the outlet of the said first part of the main heatabsorption section to the recirculating circuit, and

(e) a superheater conduit connected to the outlet of the said secondpart of the main heat absorption section.

References Cited by the Examiner UNITED STATES PATENTS 3,135,251 6/1964Kane 122-406 3,185,136 5/1965 Cozza 122-406 3,213,835 10/1965 Egglestone122406 KENNETH W. SPRAGUE, Primary Examiner.

1. A SUPERCRITICAL PRESSURE BOILER HAVING A COMBUSTION CHAMBER,COMPRISING (A) A MAIN HEAT ABSORPTION SECTION HAVING AN INLET HEADER ATITS LOWER END AND AN OUTLET HEADER AT ITS UPPER END, THE SECTION BEINGLOCATED IN THE COMBUSTION CHAMBER AND MEANS TO HEAT SAID SECTION WHEREBYONE PART RECEIVES LESS HEAT THAN ANOTHER PART AND DELIVERS FLUID OFLOWER THAN AVERAGE TEMPERATURE TO A PORTION OF THE OUTLET HEADER, (B) ARECIRCULATING CIRCUIT INCLUDING A MIXING DEVICE JOINING THE SAID INLETAND OUTLET HEADERS, (C) A FEED CIRCUIT INCLUDING A MAIN FEED PUMPCONNECTED TO THE MIXING DEVICE, (D) A RECIRCULATING SYSTEM CONNECTED INTHE RECIRCULATING CIRCUIT, (E) MEANS CONNECTING SAID PORTION OF THE SAIDOUTLET HEADER RECEIVING FLUID OF LOWER THAN AVERAGE TEMPERATURE TO THEMIXING DEVICE, AND (F) A SUPERHEATER CONDUIT CONNECTED TO A PORTION OFTHE OUTLET HEADER RECEIVING FLUID OF HIGHER THAN AVERAGE TEMPERATURE.